Over-the-air synchronization of radio nodes in a radio access network

ABSTRACT

A method is provided for synchronizing timing in phase and frequency of clocks associated with a plurality of radio nodes (RNs) in a small cell radio access network (RAN) having an access controller operatively coupled to each of the RNs. In accordance with the method, a donor list is generated for each given RN in the RAN. The donor list represents an ordered list of potential wireless access points that are able to serve as a source of a wireless sync signal for the given RN. The donor lists are distributed to the respective RNs. An access point is selected by each of the RNs from their respective donor lists to use as a sync signal source. Each of the RNs synchronize their respective clocks in phase and frequency using wireless sync signals received from the respective selected access points.

PRIORITY APPLICATION

This application is a continuation of U.S. Application No. 17/403,237,filed Aug. 16, 2021, which is a continuation of U.S. Application No.16/774,916, filed Jan. 28, 2020, now U.S. Pat. No. 11096136, whichclaims the benefit of priority under 35 U.S.C. § 119 of U.S. ProvisionalApplication Serial No. 62/798,560, filed Jan. 30, 2019, the content ofwhich is relied upon and incorporated herein by reference in itsentirety.

BACKGROUND

Packet-based timing methods are important for delivering timing overpacket-switched networks. In particular, the Precision Timing Protocol(PTP) specified in IEEE 1588-2008 is becoming a defacto standard fordelivering timing information (time/phase/frequency) from a Grand Master(GM) clock to slave clocks in end application-specific equipment such aswireless base stations providing mobile telephony services, whichrequire precise timing. The Grand Master clock provides timinginformation over the packet-switched network to the slave clocks byexchanging packets with embedded time-stamps related to thetime-of-arrival and time-of-departure of the timing packets. The slaveclock utilizes this information to align its time (and frequency) withthe Grand Master. The Grand Master is provided an external reference toserve as the basis for time and frequency. Most commonly this referenceis derived from a Global Navigation Satellite System (GNSS) such as theGPS System that in turn is controlled by the US Department of Defenseand its timing controlled very precisely and linked to the US NavalObservatory. Time alignment to the GPS clock is, for all practicalpurposes equivalent to time alignment to UTC.

In some cases the wireless base stations may include a GNSS (e.g. GPS)receiver. The GNSS receiver utilizes the available radio frequencysignals from the GNSS satellites and from that determines its position(e.g. latitude/longitude/height) as well as time. From this the receivercan generate a timing signal, together with a messaging channel carryinga data stream comprising the time-of-day at the defining pulse-edge(signal transition) of the signal. This combination of event signal andmessaging channel is referred to as 1PPS+ToD. The backhaul channelwhereby the small cell connects with the network can still be used tocarry packet-based timing signals (e.g. PTP) and this can be used as aback-up to generate timing for the small cell when the GNSS signal isinterrupted for any reason.

Operators of mobile systems, such as Universal Mobile TelecommunicationsSystems (UMTS) and its offspring including LTE (Long Term Evolution) andLTE-Advanced, are increasingly relying on wireless small cell radioaccess networks (RANs) in order to deploy indoor voice and data servicesto enterprises and other customers. However, when the small cells aredeployed indoors a built-in GNSS antenna may not have adequate signalstrength or quality to provide a good timing solution.

SUMMARY

In accordance with one aspect of the present disclosure, a method isprovided for synchronizing timing in phase and frequency of clocksassociated with a plurality of radio nodes (RNs) in a small cell radioaccess network (RAN) having an access controller operatively coupled toeach of the RNs. In accordance with the method, a donor list isgenerated for each given RN in the RAN. The donor list represents anordered list of potential wireless access points that are able to serveas a source of a wireless sync signal for the given RN. The donor listsare distributed to the respective RNs. An access point is selected byeach of the RNs from their respective donor lists to use as a syncsignal source. Each of the RNs synchronize their respective clocks inphase and frequency using wireless sync signals received from therespective selected access points.

This Summary is provided to introduce a selection of concepts in asimplified form. The concepts are further described in the DetailedDescription section. Elements or steps other than those described inthis Summary are possible, and no element or step is necessarilyrequired. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended foruse as an aid in determining the scope of the claimed subject matter.The claimed subject matter is not limited to implementations that solveany or all disadvantages noted in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one example of a mobile telecommunications environment thatincludes an enterprise in which a small cell RAN is implemented.

FIG. 2 shows a functional block diagram of one example of an accesscontroller such as the Spidercloud services node.

FIGS. 3 a and 3 b show a directed REM graph that includes a root nodethat is a macro cell serving as the root sync source.

FIG. 4 is a schematic diagram of an exemplary computer system that canbe implemented for a radio cell of a RAN or a user mobile communicationsdevice that may be configured to facilitate an automatic cell discoveryprocess for a target RAN to discover a source RAN cell by the source RANcell initiating a fake handover request to the target RAN, wherein thecomputer system is adapted to execute instructions from an exemplarycomputer readable link.

DETAILED DESCRIPTION Overview

One type of small cell radio access network (RAN) architecture that iscurrently deployed includes a network of radio nodes connected to acentralized access controller or aggregation node. One example of such acontroller or node is the Services Node available from Spidercloud,Wireless Inc. The centralized access controller or aggregation node maybe implemented as an enterprise premise-based controller element thatcoordinates the radio nodes (RNs) in the network. In an LTE embodiment,the access controller functions as a local, premises-based gateway thatanchors and aggregates a group of LTE RN_(S). One particular example ofsuch an access controller is the Spidercloud Services Node. Detailsconcerning the Spidercloud Services Node may be found in U.S. Pat. No.8,982,841, which is hereby incorporated by reference in its entirety.

Since the radio nodes and the centralized access controller are oftendeployed in an indoor environment, it is desirable to synchronize thetiming of the clocks in all of the radio access nodes in both frequencyand phase without the need for an external clock source, a GNSS source,or a wired link. That is, it is desirable to synchronize the radiooscillators in all the radio access nodes using wireless signals. Asdescribed in more detail below, in some embodiments the radio accessnodes in a small cell RAN are synchronized in both frequency and phaseusing LTE signals (in the case of an LTE network) or UMTS signals (inthe case of a UMTS network).

Operating Environment

FIG. 1 shows one example of a mobile telecommunications environment 100that includes an enterprise 105 in which a small cell RAN 110 isimplemented. The small cell RAN 110 includes a plurality of radio nodes(RN_(S)) 1151...115N. Each radio node 115 has a radio coverage area(graphically depicted in the drawings as hexagonal in shape) that iscommonly termed a small cell. A small cell may also be referred to as afemtocell, or using terminology defined by 3GPP as a Home Evolved Node B(HeNB). In the description that follows, the term “cell” typically meansthe combination of a radio node and its radio coverage area unlessotherwise indicated. A representative cell is indicated by referencenumeral 120 in FIG. 1 .

The size of the enterprise 105 and the number of cells deployed in thesmall cell RAN 110 may vary. In typical implementations, the enterprise105 can be from 50,000 to 500,000 square feet and encompass multiplefloors and the small cell RAN 110 may support hundreds to thousands ofusers using mobile communication platforms such as mobile phones,smartphones, tablet computing devices, and the like (referred to as“user equipment” (UE) and indicated by reference numerals 1251-125N inFIG. 1 ).

The small cell RAN 110 includes an access controller 130 that managesand controls the radio nodes 115. The radio nodes 115 are coupled to theaccess controller 130 over a direct or local area network (LAN)connection (not shown in FIG. 1 ) typically using secure IPsec tunnels.The access controller 130 aggregates voice and data traffic from theradio nodes 115 and provides connectivity over an IPsec tunnel to asecurity gateway SeGW 135 in an Evolved Packet Core (EPC) 140 network ofa mobile operator. The EPC 140 is typically configured to communicatewith a public switched telephone network (PSTN) 145 to carrycircuit-switched traffic, as well as for communicating with an externalpacket-switched network such as the Internet 150.

The environment 100 also generally includes Evolved Node B (eNB) basestations, or “macrocells”, as representatively indicated by referencenumeral 155 in FIG. 1 . The radio coverage area of the macrocell 155 istypically much larger than that of a small cell where the extent ofcoverage often depends on the base station configuration and surroundinggeography. Thus, a given UE 125 may achieve connectivity to the network140 through either a macrocell or small cell in the environment 100.

As previously mentioned, the access controller shown above may be theSpidercloud Services Node, available from Spidercloud Wireless, Inc.FIG. 2 shows a functional block diagram of one example of an accesscontroller 210 such as the Spidercloud services node. The accesscontroller may include topology management, self-organizing network(SON), a services node mobility entity (SME), an operation,administration, and management (OAM) module, a PDN GW/PGW module, a SGWmodule, a local IP access (LIPA) module, a QoS module, and a deep packetinspection (DPI) module. Alternative embodiments may employ more or lessfunctionality/modules as necessitated by the particular scenario and/orarchitectural requirements.

Over-The-Air Synchronization

In one aspect, the techniques described herein allow the RNs 115 in theRAN 110 to synchronize one another in frequency and phase bytransmitting and receiving wireless signals among themselves. This canbe accomplished by coordinating the radio nodes to radiate and listen atthe right moment and select an appropriate source to synchronize with.

As used herein the term sync donor refers to a radio node transmittingwireless signals that serve as a synchronization source (i.e., afrequency and/or time reference) for the clock or radio oscillator ofanother radio node, which is referred to as a sync receiver.

As used herein the term donor list refers to an ordered list ofpotential sync donors stored by the RNs.

As used herein the term sync tree refers to a tree span from a directedradio environment monitoring (REM) graph obtained from a REM scan.

As used herein the term muting pattern refers a pattern of radio nodesin the sync tree that are muted (prevented from transmitting) for aperiod of time in order to avoid interference of parent nodes by childnodes in the sync tree.

As used herein the term hop ID refers to the logical distance, measuredin units of hops, from a sync receiver to the root node that serves asthe master synchronization source. For example, a sync receiver has ahop ID of 2 hops if its sync donor is itself synchronized to the mastersynchronization source.

FIG. 2 shows is a functional block diagram showing the pertinentcomponents of one example of the access controller 210 (e.g., accesscontroller 130 in FIG. 1 ) in communication with the pertinentcomponents of one of the radio nodes 220. As shown, the accesscontroller 210 includes a cell manager 212 that incorporates a syncmanager 214. The sync manager 214 is responsible for the global state ofall the radio nodes in the RAN. Among other things, the sync manager 214distributes information received from individual radio nodes to theother radio nodes in the RAN when necessary. In general the sync manager214 is responsible for those aspects of the over-the-air synchronizationprocess that are not latency sensitive.

The cell agent 222 is the primary component in the radio node 220 thatcommunicates with the cell manager 212. The radio node 220 also includesa sync agent 224 that communicates with and operates in cooperation withthe sync manager 214 in the access controller 210. The sync manager 214and sync agent 224 communicate with one another via the cell manager 212and the cell agent 222. The cell agent 222 is responsible for thoseaspects of the over-the-air synchronization process that are latencysensitive.

The sync agent 224 communicates with a number of conventional modules inthe radio node 220, including the network listen controller 226, the RFfront-end module 228, the sync module 230 and the LTE L1/L2 module 232.The LTE L1/L2 module 232 manages the layer 1 (the physical layer) andlayer 2 (MAC layer) functionality in the LTE air interface protocolstack. The sync agent 224 communicates with the low level network listencontroller 226 and the RF front-end module 228 to tune the RF and alsodecode and analyze the LTE signals from the LTE L1/L2 module 232. Amongother things, the sync agent 224 will mute RF transmission in accordancewith the mute pattern whenever the radio node is attempting to obtain async signal or when the ancestors of the radio node in the sync tree arelistening.

The sync manager 214 collects up-to-date information from the RNs andsends the information to all neighboring RNs. Such information includesthe donor list that is maintained for each RN. Other importantinformation that is received and distributed by the sync manager 214specifies the quality of the sync between the radio nodes and theirrespective donor nodes (whether another radio node or a macro cell).After obtaining a sync tree (discussed in more detail below), the syncmanager 214 sends a donor list to each radio node. The radio nodes maylocally store the donor list in the cache associated with its sync agent224. The sync manager 214 also maintains the sync status of each radionode and receives updates of sync donors’ status from each radio node’sperspective.

The sync manager 214 also implements a number of algorithms forcoordinating the over-the-air synchronization process. For instance, onesuch algorithm assigns a score to the quality of synchronization sourcesand donors. In some embodiments the scoring may be accomplished usingthe results of conventional REM scans. The REM scan allows radio nodesto monitor and characterize their neighboring nodes. During each scan,one RN in the small cell RAN transmits at its maximum power and all theother RNs determine the power received from that transmitting RN. Thisprocess is repeated until every RN has scanned every other RN. Amongother things, the results of these measurements provide the transmittedand received powers between each pair of RNs in the small cell RAN. Insome embodiments the scoring framework may take into account measurementparameters such as RSRP, RSRQ for LTE, RSCP, RSSI, Ec/Io for UMTS. Thescoring algorithm uses all this data as input metrics that are mapped tovalues that are used to rank the quality of the donors. The mapping thatis performed may be a linear or non-linear mapping. One example of ascoring algorithm that may be employed will be presented below afterfurther discussing the functionality of the sync manager and sync agent.

The sync manager 214 uses the data from the REM scan to construct anetwork topology. From this the sync manager 214 generates a directedgraph having nodes corresponding to the radio nodes. In particular,conventional graph theory may be used to expand a directed REM graph toa sync tree using a minimum spanning tree algorithm. In this way thesync tree generally has a reduced number of edges relative to thedirected REM graph and no cycles. In one embodiment the depth of thesync tree from the root node should be as small as possible because asthe number of hops that are required from the root node to any givenchild node increases, the resulting sync quality will generallydecrease. However, a competing factor that needs to be taken intoaccount is the RSRP (reference signal received power) or the SNR(signal-to-noise ratio) of the sync signal since there may be a tradeoffbetween depth and SNR. For instance, there may be cases where a smallersync tree depth gives rise to a lower SNR. This tradeoff can beaccomplished by assigning suitable weights to each edge in the spanningtree algorithm. That is, the weights may be a function of the RSRP orSNR and the distance in units of hops.

Once the sync tree has been obtained, the sync manager 214 can generatea donor list for each radio node. The donor list specifies an orderedlist of neighboring radio nodes that can be used as sync sources,ordered from the most desirable sync source to the least desirable syncsource. The sync manager 214 can also use the sync tree to coordinatethe booting order of the RNs according to the sync tree. Specifically,in one embodiment those RNs closest to the root sync source in terms ofthe number of hops should be booted up first and the RNs most remotefrom the root sync source in terms of the number of hops should bebooted up last. By coordinating the boot order in this mannersynchronization can be achieved as soon as possible. In anotherembodiment the boot order may be determined in accordance with a scoringframework that takes into account the quality of the signal receivedfrom the root node (e.g., a macro cell) in addition to, or instead of,the logical distance in units of hops from the root node.

In some embodiments the boot-up process may be an asynchronous processso that the any radio node can be boot up at any time. In addition, thesync manager 214 may use only the local neighbor topology of a givenradio node when determining when the given radio node should boot-up. Inthis way it is not necessary to re-boot all the radio nodes in thesystem when a given radio node changes its state.

The sync manager 214 can also implement a variety of other algorithmsthat perform tasks such as coordinating the muting pattern for each RNin the network to improve the reliability of obtaining synchronization,for instance. Other tasks that the sync manager performs includesresponding to the sync agent’s donor updates and consolidating andsending updated sync trees to all sync agents.

The sync agents 224 in the radio nodes cooperate with the sync manager214 to keep close track of the current sync status and send updates tothe sync manager. The updates that are periodically provided on thecurrent sync status may include, for instance, such information as acurrent estimate of timing and frequency errors, the RSRP of the syncdonor and any estimates of interference when listening to a sync donor.The sync agents 224 may also be required to rapidly respond when thereis a change in the RF environment or sync status. Additionally, based onthe information received from the sync manager, the sync agents 224 mayneed to determine muting patterns to avoid interference.

The sync agents 224 are also responsible for quickly reselecting a syncdonor from their respective donor lists when losing sync and informingthe sync manager 214 of the change. The sync agents 224 may runpredictive analyses to determine whether the RN is about to lose syncbased on LTE layer 1 measurements and historical data so that action maybe taken before sync is lost. In some embodiments the predictiveanalysis assessing sync quality to determine if the current sync donorshould be changed may employ such techniques as linear or non-linearregression, outlier filtering, averaging, etc. In making thisdetermination it should be recognized that there is a tradeoff betweenfalse alarms warning that the sync donor should be changed and failureto change sync donors before sync is lost because the cost of a misseddetection may be very high. While losing sync will cause significantinterference, switching sync donors too rapidly could also lead to aloss of sync as well.

In some cases the muting pattern imposed by the sync agents 224 isvendor specific and thus is coordinated with the MAC scheduler to avoidscheduling data for transmission during a muted period. The mute patternis coordinated across the RNs to avoid interference between the RNs.

FIGS. 3 a and 3 b show a directed REM graph that includes a root node310 that is a macro cell serving as the root sync source. The REM graphincludes radio nodes 3201, 3202, 3203, 3204 and 3205. Radio node 3205 iscurrently not synced and needs to determine if neighboring radio nodes3203 or 3204 is the best sync donor. As shown by the bold arrows in FIG.3 b radio node 3203 is selected as the best sync donor because it isonly two hops away from the root node 320, whereas radio node 3204 is 3hops away from the root node 320. This decision assumes, of course, thatall other pertinent parameters such as RSRP or SNR are equal for bothradio nodes 3203 and 3204.

One example of a scoring algorithm that may be employed to rank thedonors will now be presented. A part of the synchronization acquisitionprocess involves decoding one or more of the standard subframe signalsreceived from the donors. These signals include the Primary Sync Signal(PSS), the Secondary Sync Signal (SSS), and the Cell Reference Signal(CRS) of the donor cell. Because the CRS is transmitted at more frequentintervals than the other signals, it may be the primary signal that isused for sync tracking.

For tracking donors, the scoring of each neighbor cell x is given by:

score(x) = h(x) − g(x_d)

where:

-   x_d is the distance of cell x in the sync graph from the macro cell;-   h(x) is a function that represents the signal quality for cell x    (examples of the function h(x) may include, for example, the RSSI    from cell x, the SINR from cell x, or a function of the two    metrics);-   g(x) may be a class of functions taking the form:    -   g(x) = a + bx + cx² + ... , where a, b, c, ..., are constants        (i.e., a polynomial function)    -   g(x) = a + bx + _(CX) ^(0.5) + .... (the exponents being any        fractional number g(x) alternatively could be an exponential        functions taking the form:    -   g(x) = a + e_1^(x) + e_2^(x) + ... , where a, e_1, e_2, ... are        constants    -   a special case of e_1 could be Euler’s number, i.e., exp(1)    -   g(x) could be a mixture of functions of the aforementioned        function classes. g(x) could be an adaptively changing function        according to the synchronization performance, and use a        different function class as needed.

In one particular embodiment, the ranking or ordering of cells on thedonor list may be performed in a pairwise manner using the scoringalgorithm described above. For instance, a function f(x, y) may bedefined which returns either cell x or cell y as the better donor. Witheach cell input x and y, a list of attributes associated with the cellis also passed along. The algorithm may operate as follows:

 Let best_cell = the first neighbor cell           for all neighbor _cell in neighbor cell _list:                   best_cell = f(best_cell, neighbor_cell)

Where function f is given as:

f(x, y):                    score_x = h(x) - g(d_x)                   score_y = h(y) - g(d_y)                   return argmax(score _x, score_y)

Illustrative attributes associated with each neighbor cell may include:

-   the SNR (dB), denoted as SNR_i-   the sync status (true/false), denoted as SS_i-   the wait status (true/false), denoted as WS_i-   hop id ([0:max_hop]), denoted as HOP_i-   A constant penalty for cells that are currently in the waiting    state, denoted as PEN.

The pairwise ordering process may then proceed as illustrated by thefollowing pseudocode:

deff(i, j):   if(HOP_i > HOP_j)     if WS_i is TRUE AND SNR_i > SNR_j + PEN        return i      else       return j   if(HOP_j > HOP_i)     if WS_j is TRUE AND SNR_j > SNR_i + PEN        return j      else       return i  if ( SS_i is TRUE AND SS_j is TRUE) OR (WS_i is EQUAL to WS_j)     if(snr_i > snr_j)        return i      else        return j  if(WS_i is TRUE)      return i   else      return j

FIG. 4 shows a simplified functional block diagram of an illustrativecomputer system 400 that can be included in any devices described hereinincluding those devices involved with synchronizing in frequency andphase, a radio node (RN) in a small cell radio access network (RAN). Forexample, the computer system 400 can be included in, without limitation,the RN 115 and access controller 130 in FIG. 1 , the access controller210 in FIG. 2 , the cell manager 212 in FIG. 2 , the sync manager 214 inFIG. 2 , the cell agent 222 in FIG. 2 . The computer system 400 includesa controller/processor 402 typically handles high level processing. Thecontroller/processor 402 may include one or more sub-processors404(1)-704(N) or cores that are configured to handle specific tasks orfunctions. An RF processor 406 implements various signal processingfunctions for the downlink including the lower level L1 processing. TheRF processor 406 may include one or more sub-processors 408(1)-408(R) orcores that are configured to handle specific tasks or functions. Amemory 410 is a computer-readable medium that stores computer-readablecode 412 that is executable by one or more processors including thecontroller/processor 402 and/or the RF processor 406. The memory 410 mayalso include various data sources and data sinks (collectivelyrepresented by element 414) that may provide additional functionalities.

The code 412 in typical deployments is arranged to be executed by theone or more processors to facilitate the discovery of a neighboringradio access system or cells reporting to a serving RAN. The code 412additionally enables implementation of both the dedicated PCI identityand common PCI identity using the same hardware infrastructure in agiven dual identity cell when executed. The hardware infrastructure mayalso include various interfaces (I/Fs) including a communication I/F 416which may be used, for example, to implement a link to an accesscontroller (e.g., access controller 130 in FIG. 1 , or access controller210 in FIG. 2 ), LAN, or to an external processor, control, or datasource. In some cases, a user I/F 418 may be utilized to provide variousindications such as power status or to enable some local control offeatures or settings. The RF processor 406 may be eliminated in someapplications and any functionality that it provides that is needed toimplement the access controller may be provided by thecontroller/processor 402.

Several aspects of telecommunication systems will now be presented withreference to access controllers, radio nodes, base stations and UEsdescribed in the foregoing description and illustrated in theaccompanying drawing by various blocks, modules, components, agentscircuits, steps, processes, algorithms, etc. (collectively referred toas “elements”). These elements may be implemented using electronichardware, computer software, or any combination thereof. Whether suchelements are implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. By way of example, an element, or any portion of an element, orany combination of elements may be implemented with a “processingsystem” that includes one or more processors. Examples of processorsinclude microprocessors, microcontrollers, digital signal processors(DSPs), field programmable gate arrays (FPGAs), programmable logicdevices (PLDs), state machines, gated logic, discrete hardware circuits,and other suitable hardware configured to perform the variousfunctionalities described throughout this disclosure. One or moreprocessors in the processing system may execute software. Software shallbe construed broadly to mean instructions, instruction sets, code, codesegments, program code, programs, subprograms, software modules,applications, software applications, software packages, routines,subroutines, objects, executables, threads of execution, procedures,functions, etc., whether referred to as software, firmware, middleware,microcode, hardware description language, or otherwise. The software mayreside on a computer- readable media. Computer-readable media mayinclude, by way of example, a magnetic storage device (e.g., hard disk,floppy disk, magnetic strip), an optical disk (e.g., compact disk (CD),digital versatile disk (DVD)), a smart card, a flash memory device(e.g., card, stick, key drive), random access memory (RAM), read onlymemory (ROM), programmable ROM (PROM), erasable PROM (EPROM),electrically erasable PROM (EEPROM), a register, a removable disk, andany other suitable media for storing or transmitting software. Thecomputer-readable media may be resident in the processing system,external to the processing system, or distributed across multipleentities including the processing system. Computer- readable media maybe embodied in a computer-program product. By way of example, acomputer-program product may include one or more computer-readable mediain packaging materials. Those skilled in the art will recognize how bestto implement the described functionality presented throughout thisdisclosure depending on the particular application and the overalldesign constraints imposed on the overall system.

1. A wireless communication system, comprising: a plurality of radionodes (RNs) each configured to serve a respective one of a plurality ofradio coverage areas; and an access controller coupled to the pluralityof RNs and configured to: generate a donor list representing an orderedlist of wireless access points among the plurality of RNs that can serveas a source of a wireless sync signal for any selected RN among theplurality of RNs; and distribute the donor list to the selected RN. 2.The wireless communication system of claim 1, wherein the selected RN isconfigured to: determine a selected wireless access point from theordered list of wireless access points to use as the source of thewireless sync signal; synchronize a respective clock in phase andfrequency based on the wireless sync signal received from the selectedwireless access point; and provide information concerning a current syncstatus to the access controller.
 3. The wireless communication system ofclaim 2, wherein the selected RN is further configured to provide theinformation concerning the current sync status to the access controllerperiodically.
 4. The wireless communication system of claim 2, whereinthe selected RN is further configured to determine a different wirelessaccess point from the ordered list of wireless access points to use asthe source of the wireless sync signal when synchronization to theselected wireless access point is lost.
 5. The wireless communicationsystem of claim 1, wherein the access controller is further configuredto assign a respective score to each wireless access point in theordered list of wireless access points to indicate a respective qualityrank when used as the source of the wireless sync signal.
 6. Thewireless communication system of claim 1, wherein the access controlleris further configured to generate the donor list by generating adirected graph having a plurality of nodes each corresponding to arespective one of the plurality of RNs.
 7. The wireless communicationsystem of claim 6, wherein the access controller is further configuredto expand the directed graph to obtain a sync tree using data obtainedfrom a radio environment monitoring (REM) scan.
 8. The wirelesscommunication system of claim 7, wherein the access controller isfurther configured to expand the directed graph using a minimum spanningtree algorithm to thereby minimize a depth of the sync tree from a rootnode serving as a master synchronization source to any child node, thedepth of the sync tree being measured in units of hops from the rootnode to any given child node.
 9. The wireless communication system ofclaim 7, wherein the access controller is further configured to:coordinate a boot order among the plurality of RNs based on the synctree; instruct a respective one of the plurality of RNs to boot up firstwhen the respective one of the plurality of RNs is closest to a rootnode of the sync tree in units of hops; and instruct the respective oneof the plurality of RNs to boot up last when the respective one of theplurality of RNs is farthest from the root node of the sync tree in theunits of hops.
 10. The wireless communication system of claim 7, whereinthe access controller is further configured to coordinate a mutingpattern to thereby prevent a respective one of the plurality of RNs fromtransmitting under any one or more of the following conditions: therespective one of the plurality of RNs is attempting to obtain thewireless sync signal; and any other one of the plurality of RNs closerto a root node in the sync tree than the respective one of the pluralityof RNs is listening for the wireless sync signal.